This invention relates generally to drive systems for electric vehicles, and more particularly to four-wheel-drive systems for electric automobiles.
The four-wheeled electric vehicles, and more particularly the electric battery-powered automobiles, normally have a conventional electric traction motor, driving two of the vehicle wheels through the final drive and differential of a drive axle. In this invention, the term "conventional traction motor" is used to indicate a traction motor of any appropriate electrical type, having a stator and a rotor. In some drive systems the conventional traction motor is integrated with the final drive and differential into a motor-transaxle. Some drive axles have a limited-slip differential for limiting the traction slip of one of the drive wheels, in case of poor traction conditions, and providing some torque to the drive wheel with better traction. The described two-wheel-drive systems are simple and efficient. However, generally, the two-wheel-drive is inferior to the four-wheel-drive, as far as the traction and handling of the vehicle are concerned.
Different four-wheel-drive systems for automotive type vehicles are known in the art. Some of these systems generate tractive force permanently throughout all four vehicle wheels, while others have provisions for selection of a four-wheel-drive mode or a two-wheel-drive mode. However, the four-wheel-drive automobiles have two drive axles, one of which is also a steering axle. In a four-wheel-drive system for electric vehicles, having a conventional traction motor, torque and rotation from the motor rotor are split between the front and rear drive axles through a transfer box and an inter-axle differential. Usually a limited-slip inter-axle differential is used for limiting the traction slip of the wheels of one of the axles, in case of poor traction conditions, and providing some torque to the wheels of the axle with better traction. However, the addition of a transfer box and a limited-slip inter-axle differential complicates significantly a four-wheel-drive system. High cost and low mechanical efficiency are the major disadvantages of the known four-wheel-drive systems, in comparison with the two-wheel-drive ones. An improvement of the average mechanical efficiency of a four-wheel-drive is achieved when the system provides for selection of four-wheel-drive mode or two-wheel-drive mode. Then, the four-wheel-drive is used normally at lower speed and bad road conditions when more traction is needed, and the two-wheel-drive is used normally at higher speed and good road conditions when the traction of two drive wheels is sufficient. However, the addition of a mechanism for selectively connecting or disconnecting one of the axles to or from the power train further complicates the four-wheel-drive systems.
It is well known in the art that the capacity for energy storage of the present-days electric batteries is quite limited, while their weight is significant. Therefore, the efficiency of the drive train of the electric battery-powered vehicles is of an utmost importance. Along with the high cost, the low efficiency is the major reason why the four-wheel-drive system of the above described types have not found practical application in the contemporary electric automobiles.
Some four-wheel-drive electric vehicles have two conventional traction motors, each one driving one of the two drive axles of the vehicle. This type four-wheel-drive systems do not require a transfer box and a limited-slip inter-axle differential. Each of the two mechanically independent drive trains of the system is simple and efficient. Nevertheless, such a system requires a quite complicated double-motor controller, capable to coordinate continuously the rotational behavior of both motors. Overall, the incorporation of two traction motors and a complex double-motor controller makes such four-wheel-drive systems quite expensive. That is why they have found limited practical application, mainly in the design of heavy electric battery-powered vehicle.
All of the above described two-wheel-drive and four-wheel-drive systems normally have rigid mechanical power trains providing a single rotational speed reduction ratio between the rotor of the traction motor and the vehicle drive wheels. Therefore, the rotational speed of the vehicle wheels, and consequently--the speed of the vehicle, is substantially directly proportional to the rotational speed of the motor rotor. The vehicle speed and tractive effort are controlled, by the operator of the vehicle, through the traction motor electrical controller, which provides variable torque and rotational speed of the motor rotor in relation with the position of the vehicle accelerator pedal and the external forces resisting or helping the motion of the vehicle. The vehicle direction of motion is controlled also through the traction motor electrical controller, which changes the direction of rotation of motor rotor in relation with the position of a forward/reverse switch, controlled by the operator of the vehicle. Usually the traction motor is also used for electric braking of the vehicle, and particularly for speed retardation on a long downhill incline, by generating torque resisting the motion of the vehicle. The braking action of the traction motor is controlled directly by the operator of the vehicle through suitable means. Usually, the traction motor controller is also arranged to protect automatically the rotor from over-speeding, if, for some reason, the power train is interrupted and the energized motor remains under no torque resisting the rotation. However, the described above electric drive control is well known and relatively simple and efficient. Nevertheless, the existing electric vehicles, having a rigid drive train, provide substantially lower maximum speed than the maximum speed of the compatible automobiles powered by internal combustion engines. That is because the current-torque-speed characteristics of the conventional traction motors of different electrical types usually cannot meet both tractive effort and speed requirements of the contemporary automobiles. A gear-shift transmission may be incorporated into the drive train for providing several speed ratios, and thus to increase the vehicle speed range. Nevertheless, such a solution is not practically applicable because of a substantial additional cost and mechanical losses, which a gear-shift transmission would contribute to the drive system of an electric vehicle.
Drive systems for electric vehicles utilizing a traction motor having two rotors and no stator, although very seldom used, are also known in the art.
The electric motors having an outer rotor and an inner rotor are known under different names, such as stator-less motors, or dual-rotor motors, or double-rotor motors, etc. However, hereinafter the term "dual-rotor motor" is accepted and used in this invention to designate such a motor. In the dual-rotor motors, usually the outer rotor is arranged as a motor field and the inner rotor is arranged as a motor armature. The inner rotor is mounted coaxially inside the outer rotor. Both rotors are mounted in the motor housing, and, when the motor is energized, rotate in opposite directions under the action of the same electromagnetic forces. Normally, the electric current is conducted from terminals on the stationary motor housing to the rotating outer rotor through the contact of brushes, attached to the motor housing, and rings, mounted on the outer rotor. Otherwise, the arrangement, and particularly the wiring and electrical control of a dual-rotor motor, is generally the same as those of a conventional motor of the same electrical type. A dual-rotor traction motor may be a direct-current motor or an alternating-current motor of any particular type and design appropriate for a traction motor of a vehicle. In the known drive systems, usually the dual-rotor motor is integrated with two planetary reducers in a motor-transaxle. Torque and rotation are transmitted between the shaft of the outer rotor and the shaft of one of the drive wheels through one of the planetary reducers, and between the shaft of the inner rotor and the shaft of the other drive wheel--through the other planetary reducer. The two planetary reducers have the same reduction ratio. The output shaft of one of the reducers rotates in the same direction as the direction of rotation of the respective rotor, while the output shaft of the other reducer rotates in direction opposite to the direction of rotation of its respective rotor. Therefore, the output shafts of both reducers, and thus the two drive wheels, rotate in the same direction. A remarkable feature of a dual-rotor motor is that both rotors, driven by the same electromagnetic forces to rotate in opposite directions, produce equal torques on their output shafts but may rotate with different absolute rotational speeds, referring to a static body--such as the motor housing. Thus, the dual-rotor traction motor in the described above motor-transaxles provides for a speed differential between the two wheels of the axle. One of the major disadvantages of the existing drive systems with dual-rotor motors is that they have generally the same quite limited speed range as the described before systems with conventional traction motor and rigid drive train. For that reason, two-wheel drive systems having a dual-rotor traction motor have been used mainly in slow-moving vehicles, such as industrial trucks. However, although a dual-rotor traction motor is more complicated than an equivalent conventional traction motor, the described above drive systems utilizing a dual-rotor motor are overall relatively simple and efficient.
When a dual-rotor motor is incorporated in a drive system, the total available torque on the shafts of both rotors is two times bigger than the torque available on the shaft of an otherwise equivalent conventional motor. Another important feature of the dual-rotor motors is that, if both rotors are made of materials with the same or similar strength, the smaller inner rotor can rotate safely with much higher absolute rotational speed than the safe absolute rotational speed of the larger outer rotor, because the destructive centrifugal forces are substantially proportional to the mass and the square of the radius of each rotor. On the other hand, the control of a dual-rotor motor, including the current-torque-speed control, electric braking and speed retardation of the vehicle, over-speed protection of the rotors, and thermal protection of the motor, is arranged generally in the same manner and by the same means as in a conventional traction motor of the same electrical type. In this regard, a dual-rotor traction motor is not more complicated than an equivalent conventional motor of the same electrical type. However, the unique features and advantages of the dual-rotor traction motors described above have not yet been fully explored and utilized in the design of electric automobiles. Therefore, it will be beneficial if a simple and efficient four-wheel-drive system for electric battery-powered vehicle can be provided by using a dual-rotor traction motor with a conventional single motor controller, instead of using two conventional traction motors with a complicated dual-motor controller or a single conventional traction motor but with the addition of some complicated and inefficient drive train, such as those described herein earlier.
Friction clutches of different types are used for transmitting and selectively interrupting the transmission of torque and rotation between the components of a vehicle drive train. Usually, means using fluid pressure or electrical means operate the automotive clutches. When engaged, the frictional clutch is an extremely efficient power-transmitting element. Also, hydraulic and electric brakes of different types are well known and used for selectively braking the rotation of the vehicle wheels or other rotating components of the vehicle drive train. The contemporary clutches and brakes are relatively simple and inexpensive automotive components. Therefore, it will be beneficial if a four-wheel drive system for electric vehicles can be arranged to provide a wide speed range and selection of four-wheel-drive mode or two-wheel-drive mode, utilizing only a clutch and a brake, instead of a long drive train including a gear-shift transmission, a transfer box, a limited slip inter-axle differential and a mechanism for selectively disconnecting or connecting one of the drive axles from or to the power train, which all are complicated, noisy, energy-consuming and expensive components.
Vehicle electronics, well known in the art, are capable to monitor continuously, through appropriate sensors, the rotational speed of the vehicle wheels or the rotational speed of the input shafts of the drive axles, as well as the rotational speed of other drive train components or other variable parameters, related to the rotational speed of the vehicle wheels, such as the steering angle of the vehicle steering wheels. Such an electronics processes the signals from several sensors, and, in accordance with a predetermined program, controls the operation of means whose quick reaction is of utmost importance for the performance and safety of the vehicle. For example, the electronics may control the performance of the engine of the vehicle, or an automatic gear-shift transmission, or an anti-lock braking system, or an anti-slip traction control system, or another system or combination of systems. Therefore, it will be beneficial if such an electronics may be incorporated in the design of an electric vehicle for providing automatic control of the drive system, in accordance with the rotational behavior of the drive axles and a predetermined program, and thus securing better performance and safety of the vehicle.